Image projector

ABSTRACT

In an image projector having at least one projection beam that is actuated in the raster mode and in the calligraphic mode for representing a raster component and a calligraphic component of a total image that is projected onto a display, to attain a higher image brightness and a sharper image contrast during projection in non-darkened rooms, the at least one projection beam is a laser beam ( 19 ) that is split into two linearly-polarized partial beams ( 21, 21 ′), with the two partial beams ( 21, 21 ′) being subjected to a separate modulation and deflection such that the one partial beam ( 21 ′) writes the raster component and the other partial beam writes the calligraphic component. The two partial beams ( 21, 21 ′) are projected simultaneously onto the display; the partial beams may be optically superposed prior to being projected.

FIELD OF THE INVENTION

The invention relates to an image projector of the generic type definedin the preamble to claim 1.

BACKGROUND OF THE INVENTION

Known CRT image projectors, which operate in raster and calligraphicmodes (U.S. Pat. No. 4,614,941), and are generally referred to asraster-calligraphic projectors, are used in, for example, flightsimulators for displaying a computer-generated image of the aircraftenvironment. In raster mode, in which the image is written by horizontaland vertical deflections of the light beam, as in a television image,the actual environment scenario is represented with all of the details,such as the tower, landing strip, houses, roads, trees and the like; thecalligraphic mode, in which the light or electron beam can be moved inany direction and at any speed, from non-movement to a high-speed pivot,permits the simultaneous display of the very bright runway lighting andcolored regions within the environment scenario, resulting in extremelyrealistic displays of the airfield and its surroundings, as well as thesurrounding landscape.

The light flux of raster-calligraphic CRT projectors is limited by thecathode-ray tube (CRT), and cannot increase significantly, so the imageprojection for representing a sufficiently bright simulated image isperformed in darkened rooms. In raster-calligraphic CRT projectors, theraster component and the calligraphic component of the total image arewritten one after the other. This limits the image-repetition rate ofthe projector. If a large number of calligraphically-displayed lights(runway lighting) is displayed, or the image resolution is very high,the raster component must be displayed in an interlaced manner, in whichcase the lights are represented calligraphically between the twohalf-images. The use of half-images leads to flickering of the totalimage.

Laser projectors possessing a considerably higher light flux are usedfor projecting significantly brighter images that also have adequatecontrast and brightness under daylight conditions. Currently, however,these laser projectors can only be operated strictly as rasterprojectors.

It is the object of the invention to render a raster-calligraphic imageprojector of the type mentioned at the

The light flux of raster-calligraphic CRT projectors is limited by thecathode-ray tube (CRT), and cannot increase significantly, so the imageprojection for representing a sufficiently bright simulated image isperformed in darkened rooms. In raster-calligraphic CRT projectors, theraster component and the calligraphic component of the total image arewritten one after the other. This limits the image-repetition rate ofthe projector. If a large number of calligraphically-displayed lights(runway lighting) is displayed, or the image resolution is very high,the raster component must be displayed in an interlaced manner, in whichcase the lights are represented calligraphically between the twohalf-images. The use of half-images leads to flickering of the totalimage.

In a known image projector of the type mentioned at the outset (U.S.Pat. No. 5,582,518), the partial beam that writes the raster componentis generated by a CRT, and the partial beam that writes the calligraphiccomponent is generated by a laser. After the two partial beams have beenmodulated and deflected appropriately, they are guided by asemi-transparent mirror to a fish-eye lens that images the two separateimages together on a spherical projection display. In a modifiedembodiment of this image projector, the CRT is replaced by a secondlaser, and the two image components of the projected image are writtenin the calligraphic mode.

In a known arrangement for generating polarized light (U.S. Pat. No.5,073,830), a non-polarized light beam that is emitted by a lightsource, e.g., an HeNe laser, is split by a polarization beam splitterinto two polarized partial beams having a half-brightness, and whosepolarization planes are rotated by 90° relative to one another. The onepartial light beam is guided directly to a lens, and the other partialbeam is guided to the lens via a 90° deflection mirror and a λ/2 plate;the lens focuses them onto a common spot. The light source that isformed in this way and radiates polarized light is used for, forexample, a video projector having a “liquid-crystal” display.

In a known image projector (WO 99/12358), after the laser beams emittedby the three lasers for the colors red, green and blue are modulated,they are guided with the image content by a light waveguide to adeflection system that images the laser beams on a display.

SUMMARY OF THE INVENTION

It is the object of the invention to render a raster-calligraphic imageprojector of the type mentioned at the outset laser-capable, soraster-calligraphically-written images can be projected with a muchgreater brightness.

The object is accomplished by the features of claim 1.

An advantage of the raster-calligraphic image projector in accordancewith the invention is that it uses a laser beam as the projection beam,and therefore has a much higher available light flux. Unlike incalligraphic CRT projectors, the splitting of the laser beam into twopartial beams allows the raster component and the calligraphic componentto be projected simultaneously, so the image-repetition in the rastercomponent is not affected by the number of calligraphically-representedlights. Therefore, high-resolution images having numerous pixels and,simultaneously, a large number of light points, are also projected“non-interlaced.” The light points in the calligraphic component can berepresented with a far sharper contrast. Their contrast to the rastercomponent results from the longer sojourn of the partial laser beam atthe light points relative to the sojourn of the other partial laser beamat a pixel of the raster line. In the use of a beam splitter that splitsthe two partial beams in a 1:1 ratio, with 1000 pixels per line in theraster component and five light points to be displayed in thecalligraphic component, the partial laser beam for the light pointremains 1000:5=200 times as long at one position. The maximum contrastfor one light point is 200:1. In the display of numerous light points inthe calligraphic component, a different splitting ratio can be selectedfor splitting the laser beam, so the high light flux compensates theshorter sojourn of the partial laser beam at the individual lightpoints. Guiding together the separately-modulated partial beams ofdifferent polarities permits a virtually loss-free superposing of thetwo partial beams in the projection head.

Advantageous embodiments of the image projector in accordance with theinvention, and advantageous modifications and embodiments of theinvention, ensue from the further claims.

In accordance with an advantageous embodiment of the invention, thepolarization directions of the polarized partial beams are rotated 90°relative to one another. The use of differently-polarized partial beamsin accordance with the invention permits the virtually loss-freesuperposing of the two partial beams in the projection head.calligraphic component, the partial beam for the light point remains1000:5=200 times as long at one position. The maximum contrast for onelight point is 200:1. In the display of numerous light points in thecalligraphic component, a different splitting ratio can be selected forsplitting the laser beam, so the high light flux compensates the shortersojourn of the partial laser beam at the individual light points.

Advantageous embodiments of the image projector in accordance with theinvention, and advantageous modifications and embodiments of theinvention, ensue from the further claims.

In accordance with an advantageous embodiment of the invention, thepolarization directions of the polarized partial beams are rotated 90°relative to one another. The use of differently-polarized partial beamsin accordance with the invention permits the virtually loss-freesuperposing of the two partial beams in the projection head.

In accordance with a preferred embodiment of the invention, a λ/2 plateis positioned in the beam path of one of the two partial beams forrotating the polarization directions.

In accordance with a preferred embodiment of the invention, a λ/2 plateis positioned in the beam path of one of the two partial beams forrotating the polarization directions.

In accordance with a preferred embodiment of the invention, themodulated partial beams are coupled into glass fibers that maintain thebeam polarization, and are supplied to a projection head by beingcoupled back out of the glass fibers, and optically superposed forprojection onto a display. This division of the image projector into alaser component and a modulation component, on the one hand, and aprojection component, on the other hand, permits a spatial separation ofthe two components, which is advantageous in an application in a flightsimulator, because only the lower-weight projection head must bedisposed on the mobile part of the simulator; the weight of the mobilepart can therefore be kept low.

The invention is described in detail below by way of an exemplaryembodiment illustrated in the drawing. Shown are in:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a block diagram of a raster-calligraphic image projector having alaser and electronics component and a projection head that can bespatially separated therefrom;

FIG. 2 a schematic, detailed representation of the optical beam paths inthe laser and electronics component in FIG. 1;

FIG. 3 a schematic, detailed representation of the optical beam paths inthe projection head in accordance with FIG. 1; and

FIG. 4 a schematic, detailed representation of the optical beam paths inthe laser and electronics component in a color projector.

DETAILED DESCRIPTION OF THE INVENTION

The image projector shown schematically in FIGS. 1 through 3 has a laserand electronics component 10 and a projection head 11, which can bespatially separated from this component; the two are connected by twoglass fibers 12, 13, and a signal line 14 and a current-supply line 15.The laser and electronics component 10 has a power supply 16, which hasa 220-Volt current supply, electronics 17 and an optical component thatwill be described in detail below.

The image projector operates in raster mode and calligraphic mode, witha raster component and a calligraphic component of a total image beinggenerated separately and projected onto a display. The optical componentof the image projector is outlined in FIG. 2. A polarized laser beam 19generated by a laser source 18 is split into two partial beams 21 and21′ in a beam splitter 20. The splitting ratio is preferably 1:1, but adifferent ratio can be selected for specific applications. Each partialbeam 21 and 21′ passes through a modulation branch 28 and 28′,respectively. The raster component R of the total image is generatedwith the partial beam 21, while the calligraphic component K isgenerated with the partial beam 21′. The polarized light of the partialbeam 21 first passes through a polarizer 22, whose polarizationdirection coincides with that of the polarized partial beam 21, and ismodulated according to the image content in a downstream electro-opticalmodulator 23. The double-headed arrows and dots shown in the beam pathsymbolize the polarization direction or polarization plane of the light.The polarization plane is the plane in which the polarized lightoscillates and propagates. In FIGS. 2 and 3, this is the drawing plane,and the plane extending perpendicular to this plane, respectively. Thetotal image to be projected is generated in a so-called image generator24, which correspondingly controls the electronics 17, which in turnactuates the electro-optical modulator 23 and the electro-opticalmodulator 23′. The modulators 23 and 23′ are configured such that, whenthe maximum modulation voltage is applied, the polarization plane orpolarization direction of the partial beam 21 or 21′ rotates by 90°. Themodulation is effected such that the maximum voltage is applied to themodulator 23 or 23′ for the maximum brightness of a pixel. Disposedbehind the modulator 23 is a second polarizer 25, whose polarizationplane is oriented perpendicular to that of the first polarizer 22.

The second partial beam 21′ of the laser 18′ is deflected after the beamsplitter 20 with a deflecting prism 26, and fed into the modulationbranch 28′. Here, the partial beam 21′ passes through the same opticalstructural component, and in the same manner, as the partial beam 21;corresponding structural components of the modulation branch 28′ aretherefore provided with the same reference characters and distinguishedfrom the optical structural components in the modulation branch 28 by aprime symbol.

The partial beams 21, 21′ exiting the second polarizer 25 and 25′,respectively, are linearly polarized in the same polarization plane. Topermit a later loss-free, optical superposing of the two partial beams21, 21′ in the projection head 11, the polarization plane of one of thetwo partial beams, here the partial beam 21, is rotated by 90°, forwhich purpose a λ/2 plate 27 is disposed downstream of the polarizer 22.The two modulation branches 28 and 28′ for the raster component R andthe calligraphic component K of the total image are defined around thisλ/2 plate 27. Each partial image 21 or 21′ is coupled into one of thetwo glass fibers 12, 13 by way of an optical coupling optics 29 or 29′.

Each glass fiber 12, 13 is connected to an optical coupling optics 30 or30′ in the projection head 11 (FIG. 3). Each partial beam 21 or 21′coupled out of the glass fiber 12 or 13 passes through a polarizer 31 or31′, which serves to suppress any rotations of the polarization planesof the partial beams 21, 21′ that may be experienced in the glass fibers12, 13, and to unambiguously define the polarization plane. The partialbeam 21 is deflected horizontally in a deflection unit 32 or a scanner.This deflection corresponds to the line deflection of the partial beam21, and is performed with a correspondingly-high deflection frequency.

One possible embodiment of the deflection unit 32 is in the form of arapidly-rotating polygonal mirror. Another possible embodiment of thedeflection unit 32 is as a micro-optical mirror. The partial beam 21′,which is coupled out of the glass fiber 13 and is responsible for thecalligraphic component K, passes through the same polarizer 31′ for thesame purpose, and is deflected horizontally in a deflection unit 32′. Incontrast to the line deflection of the raster component, the horizontaldeflection of the calligraphic component can be effected slowly within aline for the light points.

One possible embodiment of the deflection unit 32′ is an electroplatedmirror that is operated such that it approaches each light point withinthe line in quick succession. In the process, it writes the linesalternately from left to right and from right to left. This avoids arapid return. Another possible embodiment of the deflection unit 32′ forhorizontally deflecting the partial beam 21′ is, for example, amicro-optical mirror. The electronics 17 controls the two deflectionunits 32 and 32′ via the signal line 14.

After the deflection units 32, 32′, the two partial beams 21 and 21′ aresuperposed in a polarization beam splitter 33, for which purpose adeflection prism 34 has already deflected the partial beam 21′ to thepolarization beam splitter 33. The superposed partial beams 21, 21′ aredeflected vertically in a further deflection unit 35. An electroplatedmirror is preferably used for this procedure. The mirror changes itsangle by a small increment for each line. After each image, it returnsto its initial position. Instead of an electroplated mirror, however, itis also possible to use a different deflection unit 35 that performs thesame action, such as a micro-optical mirror. This deflection unit 35 isalso controlled by the electronics 17 via the signal line 14. Theprojection lens 36 projects the generated raster-calligraphic image ontothe display or another projection surface.

According to the above-described principle of separate light modulationfor the raster component and the calligraphic component, a monochromaticimage having n gray stages is obtained in the color of the lasergenerated by the laser source 18. The generation of color imagesrequires three laser sources 18 having lasers of different wavelengths,as shown in FIG. 4. Each laser source 18 emits light in the red, greenand blue spectral range. The wavelengths can be, for example, 629 nm,532 nm and 446 nm. Each laser beam 19 of the three laser sources 18 issplit into the two partial beams 21 and 21′, as described in conjunctionwith FIG. 2, and passes through the modulation branch 28 or 28′. Priorto being coupled into the glass fibers 12, 13, the partial beams 21 ofall three laser beams 19 that write the raster component, and thepartial beams 21′ of all three laser beams 19 that write thecalligraphic component, are optically superposed with the aid ofdichroic mirrors 37, 37′. The dichroic mirrors 37, 37′ have differenttransmissions and reflections in the three spectral ranges of red, greenand blue. The dichroic mirrors 371 and 371′ have a high transmission forred and a high reflection for green, and the mirrors 372 and 372′ have ahigh transmission for red and green and a high reflection for blue. Onlysimple deflection mirrors 38 and 38′ are necessary for coupling in thepartial beams 21 and 21′ in the red spectral range.

After the beams have been coupled into and out of the glass fibers 12,13 in the projector head 11, the course of the beam paths of the partialbeams 21 and 21′ is as described in conjunction with FIG. 3.

What is claimed is:
 1. An image projector having at least one projectionbeam that is actuated in the raster mode and the calligraphic mode fordisplaying a raster component and a calligraphic component of a totalimage that is projected onto a display, the beam comprising two partialbeams (21, 21′) that were superposed prior to the image projection andare simultaneously projected onto the display, and are subjected to aseparate modulation and deflection such that the one partial beam (21)writes the raster component and the other partial beam (21′) writes thecalligraphic component, characterized in that the partial beams (21,21′) are two linearly-polarized laser beams that are generated by thesplitting of a laser beam (19), and have polarization directions orplanes that are rotated by 90° relative to one another, and apolarization beam splitter (33) is provided for superposing the partialbeams (21, 21′).
 2. The projector according to claim 1, characterized inthat the superposed partial beams (21, 21′) are guided via a projectionlens (36) and a vertically-deflecting deflection unit (35), preferablyan electroplated mirror, that is disposed in front of the projectionlens (36).
 3. The projector according to claim 1, characterized in that,for splitting the laser beam (19), the beam is guided by way of a beamsplitter (20), and each partial beam (21, 21′) passes through the samepolarizer (22, 22′).
 4. The projector according to claim 1,characterized in that an optical modulator (23, 23′), through which eachpartial beam (21, 21′) passes, is embodied such that, with the maximummodulation voltage, it rotates the polarization direction or plane ofthe incident, polarized partial beam (21, 21′) by 90°; a polarizer (25,25′) is disposed downstream of the modulator (23, 23′), with thepolarization direction or plane of the polarizer being rotated by 90°relative to that of the polarized partial beam (21, 21′) that isincident at the modulator (23, 23′); and the modulator (23, 23′) isactuated such that the maximum voltage is applied to the modulator (23,23′) for maximum pixel brightness.
 5. The projector according to claim4, characterized in that the polarization directions or planes of thetwo partial beams (21, 21′) are rotated in the beam path after themodulators (23, 23′), preferably by an optical component (27) that isdisposed in the beam path of one partial beam (21) and rotates thepolarization direction or plane of the incident, polarized partial beam(21) by 90°.
 6. The projector according to claim 5, characterized inthat the optical component is a λ/2 plate (27), which is preferablylocated directly behind the polarizer (25) disposed downstream of themodulator (23).
 7. The projector according to claim 1, characterized inthat each modulated partial beam (21, 21′) is coupled into apolarization-maintaining glass fiber (12, 13) by means of an opticalcoupling optics (29, 29′), and fed into a projection head (11) by way ofthe glass fiber (12, 13).
 8. The projector according to claim 7,characterized in that each glass fiber (12, 13) is connected to one oftwo coupling optics (30, 30′) disposed in the projection head (11); thepartial beam (21) that exits the coupling optics (30) and writes theraster component is guided via a deflection unit (32), preferably arotating polygonal mirror, that deflects the partial beam (21)horizontally with a high deflection frequency; and the partial beam(21′) that exits the coupling optics (30′) and writes the calligraphiccomponent is guided via a deflection unit (32′), preferably anelectroplated mirror, that deflects the partial beam (12′) horizontallywith a low deflection frequency.
 9. The projector according to claim 8,characterized in that the deflection unit (32′) for the partial beam(21′) that writes the calligraphic component is embodied such that allof the light points to be displayed within a line that is established bythe maximum horizontal deflection are actuated in quick succession. 10.The projector according to claim 9, characterized in that the deflectionunit (32′) is embodied such that the horizontal deflection direction isopposite in consecutive lines.
 11. The projector according to claim 8,characterized in that a polarizer (31, 31′) is disposed downstream ofeach coupling optics (30, 30′).
 12. The projector according to claim 7,characterized in that three projection beams are provided for generatinga total image in color, with one projection beam being a laser beam (19)in the green spectral range, one being a laser beam (19) in the redspectral range and one being a laser beam (19) in the blue spectralrange; and before the partial beams (21, 21′) are coupled into the glassfibers (12, 13), the partial beams (21) of all three laser beams (19)that write the raster component, and the partial beams (21′) of allthree laser beams (19) that write the calligraphic component, areoptically superposed.
 13. The projector according to claim 12,characterized in that dichroic mirrors (37, 37′) having differenttransmissions and reflections in the three spectral ranges are providedfor superposing the three respective partial beams (21, 21′).